The Kepler space telescope has lined up a number of planets in other star systems that might host life. But what about closer to home? Could simple life have arisen on our galactic doorstep? Caroline Morley

Jupiter's icy moon Europa is far from the habitable zone – the narrow band around a star where a planet could have liquid water. However, because of unusual features of Europa's orbit around Jupiter and its spin, changing gravitational forces probably create heat that melts water to form an ocean beneath the icy surface. Whether the moon is habitable may depend on the thickness of its ice – microbes would have to be close to the surface to access the chemicals they need.